Some types of circuits produce very little heat, and can be operated with a passive cooling technique, such as convection cooling. On the other hand, there are a variety of circuits which produce extremely large amounts of heat, and require some sort of active cooling. One example is a high-power phased array antenna system, which consumes large amounts of electrical power, typically on the order of tens of thousands of watts.
These phased array antenna systems commonly include monolithic microwave integrated circuits (MMICs), which consume substantial amounts of power, and which generate substantial amounts of heat. Pre-existing systems of this type usually include MMICs that need cooling for a thermal density of no more than about 100 Watts/in2. However, the industry trend is toward the development and use of MMIC devices which consume substantially higher amounts of power and which dissipate substantially greater amounts of heat. One example is gallium nitride (GaN) MMIC technology, and another example is gallium arsenide (GaAs) MMIC technology. In a GaN MMIC, the typical thermal density can be on the order of at least 400 Watts/in2, when the radio frequency duty cycle is at 100%. Although pre-existing techniques for cooling phased array antenna systems have been generally adequate for their intended purposes, they use a single phase coolant, such as a refrigeration-cooled polyalphaolefin (PAO). Such a pre-existing cooling system with a single-phase coolant is only capable of handling thermal densities up to about 100 Watts/in2. Consequently, these pre-existing cooling systems and techniques are not entirely satisfactory for the levels of heat dissipated by next-generation technology in phased array antenna systems.